1. Field of the Invention
The present invention relates to improving the accuracy and reliability of cold junction compensation for a thermocouple by mounting a cold junction compensation resistor in the same environment as the cold junction of the thermocouple.
2. Description of the Prior Art
A typical application of thermocouple use with a two wire current transmitter for example is shown in U.S. Pat. No. 3,680,384, issued to Grindheim. In FIG. 5 of this patent, a thermocouple circuit is illustrated, and cold junction compensation is provided.
Commercial embodiments of two wire thermocouple input two wire current transmitters have been sold by Rosemount Inc. of Eden Prairie, Minn. as their Models 441, 442, and 444 Two Wire Transmitters. A typical arrangement is shown in a 1978 product bulletin on the Model 444 product data sheet 2263. This bulletin illustrates the two wire circuit used with the thermocouple.
The two wire transmitter housing that is used with these devices has two compartments that are separated by a divider wall, with only the leads to the electronics passing through the wall. One of the compartments contains the electronics needed including the entire cold junction compensation bridge, and the other compartment houses the terminal strip and zero and span adjustment screws. The terminal strip or block in the second compartment of the two wire transmitter housing is readily accessible to the exterior by removing a cover. The terminals for connecting the thermocouple are on the terminal block, as well as the test and input power terminals.
In the devices sold to date, the cold junction compensation resistor, which is normally needed for thermocouples, and is shown in U.S. Pat. No. 3,680,384 schematically as R4, has been mounted with the rest of the cold junction compensation bridge resistors on the circuit boards, in the housing compartment separated from the terminal block. This arrangement has been used for years, without realizing that errors could be calibrated into the circuitry because of temperature differences in the two chambers during calibration when the access cover to the terminal block and zero and span adjustments were removed.
The cold junction temperature compensation resistor is generally a nickel wire wound resistor whose resistance increases with temperature, just as a thermocouple voltage does, and thus the compensation signal is dependent upon the temperature of the cold junction compensation resistor. It is generally desired that the temperature of the cold junction of the thermocouple and the temperature of the compensation resistor remain the same, so that the cold junction temperature does not affect the output of the bridge network or transmitter.
When the dual compartment housing is used, with the cold junction compensation resistor in the electronics compartment, the terminal block side of the housing may be shocked by a sudden change in temperature which is not transferred to the electronics compartment of the housing quickly enough to change the resistance of the cold junction compensation resistor during the calibration process, for example. Errors will occur unless the temperature of the compensation resistor matches that of the cold junction of the thermocouple.
In the present transmitters the sudden temperature changes most often occur during calibration of the transmitter since that is the most likely cause for removing the cover from the terminal block side of the housing. Thus, if the transmitter is being calibrated during a temperature gradient, an error is calibrated into the transmitter in the conventional arrangement when the cold junction compensation resistor is with the electronics in a different compartment from the terminal block and calibration adjustments. Such error would then be continuous until such time as the transmitter is recalibrated, which generally would not be very often.